1. Field of the Invention
The present invention relates to a surface-conduction electron-emitting device provided with an anti-static function, an electron source using it, further a picture display unit with the use of the electron source, and manufacturing processes for the same.
2. Related Background Art
In recent years, a flat panel display using a surface-conduction electron-emitting device has been developed actively. The above described electron-emitting device has a pair of device electrodes placed at a predetermined space apart from each other on an insulating substrate normally made of a glass substrate, and an electroconductive thin film arranged astride the above described device electrodes, and a fissure formed inside the above described electroconductive thin film by energization processing for the above described electroconductive thin film; and emits electrons from the above described fissure through applying voltage between the above described device electrodes. A display panel in an image display apparatus is composed of an electron source substrate that has a plurality of the electron-emitting devices and matrix wiring formed on an insulating substrate, and a light-emitting member for emitting light when irradiated with electrons emitted from the electron-emitting devices, both of which are placed so as to face each other.
Such an electron source substrate has had the problem that the electric potential of the surface of an insulating substrate becomes unstable due to the emission of electrons from an electron-emitting device, and the trajectory of a projected electron beam becomes unstable. In addition, when charged particles such as electrons and ions are injected into the surface of an insulating substrate, the substrate produces secondary electrons, but particularly under a high electric field, it causes overdischarge, and it is experimentally confirmed that the overdischarge remarkably deteriorates electron emission characteristics of a device and destroys the device in the worst case. In order to prevent the instability of the electron emission characteristics and the discharge degradation of the device in a vacuum, it is effective to cover the surface of the insulating substrate with a suitable high resistivity film so that the surface can not be exposed to the charged particles. For this reason, a conventional electron-emitting device has prevented the surface of the insulating substrate from electrostatically being charged, by covering the surface with a high resistivity film having predetermined sheet resistance. (See [Patent Literature 1] Japanese Patent Application Laid-Open No. H08-180801, [Patent Literature 2] Japanese Patent Application Laid-Open No. H11-317149, and [Patent Literature 3] Japanese Patent Application Laid-Open No. H02-060024.)
However, when a high resistivity film is formed on the whole surface of a substrate including electron-emitting devices, a leak current passes between device electrodes through the high resistivity film, and there are cases where the amperage is unexpectedly large. The factors of passing the large current include a high resistivity film more thickly formed than desired thickness, due to the uncontrollability of the thickness of the film itself. When the high resistivity film is thickly formed and acquires low sheet resistance, such a film passes a leak current through the high resistivity film itself when the device is not driven and applied voltage is low, and has caused a problem of putting a large strain on a driving integrated circuit for driving.
It was also found that a too thick high resistivity film formed on an electron-emitting device hinders the device from emitting electrons though depending on the structure of the device.
Accordingly, when the high resistivity film is provided on an electron-emitting device in order to prevent the device from being charged, the thickness of the film had to be precisely controlled. However, it has been found that it is difficult to reduce a leak current only by controlling the thickness of the high resistivity film.
The reason shall be now described below. A manufacturing process for a recent electron-emitting device includes forming a fissure which is a portion for emitting electrons, by applying voltage to an electroconductive thin film, and then activating the electroconductive thin film by applying voltage thereto in an atmosphere of a gas containing a carbon compound. The activation step forms the deposit of carbon mainly containing carbon and/or a carbon compound in the vicinity of a fissure part to increase the number of emitted electrons. If the above described high resistivity film is formed without considering the conditions of the activation step, the carbon is deposited so as to be stacked on the high resistivity film around the edge of the electroconductive thin film, consequently decreases the sheet resistance of the electroconductive thin film in the vicinity of the electron-emitting part as described above, and increases a leak current.
Those passing leak currents have caused a problem of decreasing the apparent efficiency of a device. Here, the efficiency of a device means a ratio of a current due to electrons emitted out into a vacuum (hereafter called an emission current, Ie) to a current passing when voltage is applied to a pair of the facing device electrodes of a surface-conduction electron-emitting device (hereafter called a device current If). It is desirable to minimize a device current If, and maximize an emission current Ie, but when the device is coated with a high resistivity film as described above, a leak current due to a high resistivity film is added to a device current, and lowers the efficiency. In addition, if an anti-static film is formed so as to be simply separated from an electroconductive thin film, one part of an insulating substrate is exposed and the exposed portion is charged with electricity. The electrostatic charge particularly in the vicinity of an electroconductive thin film, further particularly in the vicinity of an electron-emitting portion, wields a large influence over electrons emitted from the electron-emitting portion, and consequently tends to disorder the trajectory of emitted electrons.